Hybrid magnetic layer systems consisting of a heavy metal, an ultrathin ferromagnet (F), and an antiferromagnet (AF) are interesting material systems that potentially exhibit both the interface-driven exchange bias (EB) effect and the asymmetric exchange interaction, known as Dzyaloshinskii-Moriya interaction (DMI) 1. The DMI is essential for stabilizing chiral spin-structures like skyrmions, which are promising candidates for magnetic memory technologies 2. Due to the DMI and the resulting Néel domain walls with chirality set by DMI, tilted magnetic moments are present at the domain edges 3. Experimentally, this effect has been observed as asymmetric domain propagation while applying in-plane and perpendicular magnetic fields simultaneously 4. While the DMI and EB can be modified independently from each other 5, only few studies so far have focused on the interplay between chiral DMI and the unidirectional EB anisotropy, affecting the magnetic domain texture and the resulting magnetization reversal.
We report on a systematic study of the magnetization reversal in perpendicularly exchange biased Ti/Au/Co/NiO/Au micro stripes by high-resolution Kerr microscopy. Thereby, the magnetization reversal process is observed to be asymmetric with respect to the two branches of the hysteresis loop. We are able to quantify this as an asymmetry of the nucleation density formed in the two field branches as a function of the structure width. Additionally, a local asymmetry in the domain nucleation and domain wall movement within each stripe is observed. This phenomenon is investigated by field-cooling and the application of additional in-plane magnetic fields during the magnetization reversal. Additionally, XMCD/XMLD-PEEM experiments were performed to reveal the corresponding domain patterns in the F and AF. The results pave the way in the understanding of the interplay between chiral DMI and the unidirectional EB anisotropy in micro- and nanostructures.
References:
1 F. Hellman et al., “Interface-induced phenomena in magnetism,” Rev. Mod. Phys., vol. 89, no. 2, pp. 1–79, 2017, doi: 10.1103/RevModPhys.89.025006.

2 A. Manchon et al., “Current-induced spin-orbit torques in ferromagnetic and antiferromagnetic systems,” Rev. Mod. Phys., vol. 91, no. 3, 2019, doi: 10.1103/RevModPhys.91.035004.

3 S. Rohart and A. Thiaville, “Skyrmion confinement in ultrathin film nanostructures in the presence of Dzyaloshinskii-Moriya interaction,” Phys. Rev. B - Condens. Matter Mater. Phys., vol. 88, no. 18, pp. 1–8, 2013, doi: 10.1103/PhysRevB.88.184422.

4 P. Kuswik et al., “Asymmetric domain wall propagation caused by interfacial Dzyaloshinskii-Moriya interaction in exchange biased Au/Co/NiO layered system,” Phys. Rev. B, vol. 97, no. 2, pp. 1–7, 2018, doi: 10.1103/PhysRevB.97.024404.

5 P. Kuswik, M. Matczak, M. Kowacz, F. Lisiecki, and F. Stobiecki, “Determination of the Dzyaloshinskii-Moriya interaction in exchange biased Au/Co/NiO systems,” J. Magn. Magn. Mater., vol. 472, no. April 2018, pp. 29–33, 2019, doi: 10.1016/j.jmmm.2018.10.002.

Fig. 1: Asymmetric nucleation in 10 μm wide Ti/Au/Co/NiO/Au stripes. The Kerr images show that for the decreasing field branch a)-c) the domains are formed preferentially at one edge, smaller ones are formed equally at both edges for the increasing field branch d)-f).